Shared Neutral Between Distribution and Split Phase

I do not have an extensive background in Power. I've recently been brushing up on topics from undergrad related to power distribution and residential wiring in general. I understand that in general, houses in the US are fed a three wire, split-phase 240V power from the transformer on the utility pole. 2 of the wires are "hot" and the potential between them is 240V; the potential between either "hot" and the third wire (the grounded neutral, from the center-tapped transformer) is 120V.

I believe (and could be wrong?) that the 3-phase bus that is distributed is fed from a Wye transformer, because the utility poles are always grounded, and I can generally see 4 wires overhead (3 phases, and the grounded neutral from the center of the Wye). The phase to phase potential on this bus is generally 12-13.8kV, so a step-down transformer is needed to produce the 240V potential for the residential loads.

Here is where I get confused. From what I understand (and see the attached schematic), the primary step-down transformer is fed 1 phase and neutral (which is grounded to the earth), and the secondary is center-tapped, which is also grounded to earth.

When grounding the center tap of the secondary, how is this not seen as a ground fault on the primary side? For some reason I want to think that there is a potential difference between the secondary ground and the primary ground. If I were to take a meter and measure between the grounded primary phase and the grounded secondary phase, this should read 0V, no? For some reason, I cannot wrap my head around this.

Can someone correct me in my logic above? I'm obviously confused somewhere in my thinking, but my lack of background in this area if hindering me....and it is driving me crazy! :)

Remember that what comes out of the secondary center tap is current. An ideal wire has 0 resistance so even if current runs through the neutral there will not be any potential at that center tap. In reality, for a large and old service the neutral will have some potential because the ground connection is not great and because the grounds have been mixed up with the neutrals. Seriosly, if you get into that industry don't trust neutrals.

Ideally current will never flow into the ground. The neutral current will return to the center tap and pass through the center of the secondary coil towards one of the ends of the secondary coil. Maybe it would help if you drew in a load for your diagram. Draw a simple 120V light bulb that is hooked up past one of the meters. Next, outline a loop for the current that passes through one half of the secondary coil and through the bulb. Next outline a loop of current that passes through the primary of the center tapped transformer and the secondary of the transformer on the power pole. Even if those loops overlap it should not affect the voltage of an ideal wire.

By the way, The neutral currents that return from opposite sides of the secondary are 180 degrees different in phase. That means that they tend to cancel. In a service with good distribution you shouldn't read too much current going in or out of the center tap. You will of course read current on a single neutral that goes to one load, like an outlet. As you follow the netral wires back towars the transformer they will join together on busses in the panels. They tend to start cancelling out if you follow them back further.

Old rules in the code book used to allow electricians to use a smaller neutral wire in certain circuimstances beacuse of that cancellation but now everyone requires the neutral to have the same ratings as the hot. Different circuits coming from the panel could share a single neutral in the past.

When grounding the center tap of the secondary, how is this not seen as a ground fault on the primary side?

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Remember Kirchoff's Current Law - Current flows in closed loops.
To put that colloquially, Current must get back to where it came from. And it has no interest in going anywhere else that's not a shortcut.
Which means,
Any current leaving a transformer winding must get back to that same exact winding by hook or by crook.

The current leaving your 12.4KV wye goes through the high side winding of the "pole pig" transformer behind your house, and back to the same 12.4KV wye winding through the neutral.
That's the high side current loop.
The low side of the "pole pig" is a separate closed current loop.

Drawing the current loops in two different colors on that diagram should help.

It will also help to make that schematic drawing more closely resemble the physical layout - for as drawn it looks as if high side neutral current mixes with low side behind the meter boxes, which is certainly not the case. That's how mediocre drafting practice misleads folks.

I'd tie pole pig primary returns into that neutral wire where it's still vertical, just below the three horizontal phase lines.
That way it looks more like the top of the pole, where that connection really is.
Next i'd add an earth ground symbol just below that connection, because there's a copper wire on side of the pole that earths the neutral node there also. That assures that primary current does not have to go clear to the meter box to find earth (in the unlikely event of an open neutral wire).
After you've done that, now draw the current loop for a primary side ground fault and observe it doesn't mix with house power current loop.
And that's why it is important to call the electric company if urchins steal that copper earthing wire off the side of your power pole.

Were i any good with graphics i'd draw it.... i come from the pre-CAD days when draftsmen were half artist and strove to make drawings intuitive.

IEEE standard 142, the "Green Book" is an excellent tutorial on grounding. You'd be surprised how many EE's don't really understand it. It's very much worth having a copy in your personal library.

Thanks for the insight! I think both of you have helped clear up some confusion. However...

What really "sparked" (pun intended lol) my question is helping my brother troubleshoot a coil pack on his jeep that drives the high voltage needed for the spark plugs. I had never really looked at the schematic for the spark plugs used in cars in general, so I did a little googling and found the attached schematic.

In this schematic, it is a mutated formed of my original question: the primary inductor produces a back EMF when the switch is opened as predicted by Faraday's Law. This back EMF is "amplified" into the secondary, producing the high voltage needed to create the spark in the cylinder for combustion. However, without consulting the schematic, my intuition told me the two secondary conductors would be fed to each spark plug, as the high voltage would be produced in the "line-to-line" connection (if we can denote that in this context). However, I soon found out that it is customary for ONE coil to feed TWO spark plugs (almost like a phase to neutral connection in residential wiring), where one leg of the secondary line and the ground connection from the primary is fed to one spark plug (as seen in the attached schematic). However, the secondary doesn't appear to be grounded in this schematic.

If I try and visualize the flow of current during a "spark," current leaves the source feeding the primary, through the primary coil and back to the source. In the secondary, a second current leaves the secondary coil, but travels back to the primary side through the ground connection? This is what prompted me to ask if a ground fault current on the secondary of a transformer is "sunk" into the primary side...which appears as if that happens in the spark plug?

Forgive me if this is a trivial question, and thank you for all the help, it is very much appreciated.

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No, it's non trivial. I first encountered that spark scheme on a '49 Harley Davidson. Took me quite a while to believe it could work, yet the cycle ran fine.

Back to Kirchoff. And your schematic. Draw your two current loops, then try this talk-through.

Primary current gets interrupted by the opening of the points.
The capacitor prevents arcing at the points which is necessary if you want them to last more than about twenty minutes .
Flux decreases in proportion to current, and current collapses very quickly what with points open, so there's a fast decrease in flux.
Every turn that encircles that flux therefore sees a huge negative d[itex]\Phi[/itex]/dt .
So both coils experience an induced voltage, your "counter EMF".

The primary has not many turns so it sees modest voltage, like a few hundred volts. Touch that terminal on points if you don't believe me.
The secondary has copious turns and sees kilovolts. Enough kilovolts to jump two sparkplug gaps. As you know, don't touch that one it REALLY hurts.

So the manufacturer can save a coil by letting one coil serve two cylinders. Both get a spark once per revolution - but one of the cylinders is on exhaust stroke when there's nothing in it to burn.

Once again, the two current loops are independent. Draw them in two different colors and you'll see they do not mix.

When you see "misfire" code on your OBD reader, notice if you get it on two cylinders. If so check firing order - if the misfiring cylinders are one turn apart that's a strong indicator of coil trouble. They will swell up and crack with age and heat, then they're susceptible to water intrusion

There is a hidden problem with the series spark plug circuit. The problem is that plug performance and lifetime is dependent on the polarity of the spark. It is normal to operate a spark plug with the centre electrode as negative. In distributorless ignition systems the two plugs are consistently operated with opposite polarity. You should regularly swap over the plugs or their leads to age plugs equally.

No, it's non trivial. I first encountered that spark scheme on a '49 Harley Davidson. Took me quite a while to believe it could work, yet the cycle ran fine.

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My Citroen 2CV too. The coil was right at the front of the engine and just behind the air (and rain) grille at the front of the car. The contact breaker was on the front of the crank shaft and there was no vacuum advance. Crude and cheap as you like and it frequently let me down until I installed an electronic ignition system and the spark was 10 times better. I used to replace plugs pretty often. - but it only needed two at a time.